Aluminum is commonly used to manufacture many different articles. When compared to steel, aluminum owes its versatility as an engineering material to its easy workability, its somewhat low specific gravity, and its relative resistance to corrosion by the ambient environment.
The resistance to corrosion exhibited by aluminum is due to the formation of a substantially transparent "natural" oxide layer upon exposure to air. Unfortunately, this "natural oxide" layer does not always have a uniform thickness. Because of this, natural oxides generally are removed from aluminum products, and the product thereafter is "anodized," or controllably oxidized, to provide a protective oxide layer with better quality.
Anodizing processes generally involve the use of a bath containing an electrolyte, such as sulfuric acid, oxalic acid, chromic acid, phosphoric acid, or combinations thereof, with or without certain addition agents. The aluminum workpiece generally is used as an anode and a component made of steel or other suitable material is used as a cathode. The anode and cathode are immersed in the electrolyte solution, and a direct or alternating current is passed through the electrolyte.
Although anodizing, itself, imparts satisfactory corrosion resistance to aluminum components, anodizing also suffers from several disadvantages. One disadvantage is the porosity of the oxide formed at the surface of the aluminum component. A typical anodizing treatment results in a porous polygonal cellular microstructure superimposed on a thin (less than 100 nm) "barrier" layer. The diameter of the pores in the microstructure can be as small as 10 nm. The cell dimension can be as small as about 30 nm.
The pores formed at the surface of anodized aluminum are undesirable because they tend to serve as corrosion sites, which give rise to deep pits. Deep pits in the anodized surface often result in "blooms" or white spots on the surface of the aluminum. In order to protect anodized aluminum from corrosion, especially in halide or salt-containing environments, the pores of the aluminum oxide customarily are sealed by immersion in a hot solution containing hexavalent chromium. A complex chemical reaction occurs, forming a solid compound of chromium, aluminum, oxygen, and some hydrogen within the pores of the anodized surface. This solid compound seals the pores against penetration by corrosive agents.
Unfortunately, hexavalent chromium solutions are toxic. The use and disposal of hexavalent chromium solutions therefore creates environmental concerns. These concerns, and their associated costs, have created an urgent need for an alternative sealing process that is free from such hazards.
Some have attempted to develop alternative sealing processes using other chemical solutions. To date, these alternative chemical solutions have not been entirely successful. A non-toxic, effective method for sealing anodized aluminum surfaces is urgently needed.
Most anodizing treatments require that the aluminum component be immersed in an aqueous solution. Even after drying, a film of water molecules (about two monolayers thick) tends to remain strongly adsorbed to the anodized surface. Where the anodized surface will be treated with a relatively hydrophilic sealant, the presence of such adsorbed water molecules should not interfere with the sealing process. However, if the anodized surface will be treated with a hydrophobic sealant, the adsorbed water molecules could interfere with the sealing process, and should be removed from the surface before the sealant is applied.
The removal of water molecules from an anodized surface is not a simple matter. Water molecules are polar, and thus have a charge distribution within the molecules, themselves. The attraction between the anodized surface and the polarized water molecules creates a weak bond which holds the water molecules to the anodized surface. In order to break this weak bond, the water molecules must be provided with enough energy to break free from the anodized surface.
A number of methods exist for freeing adsorbed water molecules from various surfaces. These methods include exposing the anodized surface to: sonar energy; heat; a flow of inert gas; a beam of de-focused electrons; and, UV light.
The use of sonar energy to free adsorbed water molecules has proven to be time consuming and not entirely successful. Heating of the surface is more successful in actually desorbing the water molecules from the surface; however, not all of the adsorbed water molecules are removed by heat, and the application of heat can be cumbersome and time consuming. A flow of inert gas, such as nitrogen, removes some adsorbed water molecules; however, the movement of the gas molecules is random, and it is likely that not all of the adsorbed water molecules will be removed by the gas. Whether de-focused electrons can successfully remove adsorbed water molecules from an anodized surface is not known; however, the technique has not been used commercially.
Water molecules absorb certain wavelengths of UV light. The absorbed energy should excite the water molecules into a vibrational mode, freeing the water molecules from the surface to which they are adsorbed. However, the UV light that has been used in the past to desorb water molecules from various surfaces has been relatively high intensity, or short wavelength UV light. The conventional source of UV light is a mercury vapor lamp. In most mercury vapor lamps, essentially all radiation having a wavelength shorter than 200 nm is shut off by a silica envelope. Water has a low coefficient of absorption in the relatively short wavelength ranges produced by mercury vapor UV lamps. As a result, a relatively long period of time has been required to desorb water molecules from a surface using short wavelength UV light.
A more effective and economic method is needed for removing adsorbed water molecules from anodized surfaces. Also needed is a method for sealing an anodized aluminum surface with a medium that is chemically inert and impermeable, using a process that results in byproducts that are less hazardous to the environment than hexavalent chromium.